The present invention relates to Global Positioning System (GPS) receivers, and deals more particularly with a method and a system for improving the performance of a dual frequency civilian GPS receiver.
The Global Positioning System (GPS) is a satellite based navigation system having a constellation of 24 Earth orbiting satellites. These satellites are approximately uniformly dispersed around six circular orbits having four satellites each. Theoretically, four or more GPS satellites are visible from most points on the Earth""s surface.
Each GPS satellite presently transmits at two frequencies: L1 (1575.42 MHz) and L2 (1227.60 MHz). There exists provision (for the future) for a third frequency L5 (1176.45 MHz) as well. The L1 frequency has two different spread-spectrum codes modulated on it: a coarse acquisition (C/A) code and a Y code. The C/A code is an unclassified code intended for civilian navigation. It has a chipping rate of 1.023 MHz and a sequence length of 1023 chips. The Y code is a classified unknown code; people doing research in this area have found it to be the product of two codes: a precise (P) code and a W code. The P code is an unclassified code with a chipping rate of 10.23 MHz. The P code is long enough that it does not repeat during a week; it is reset at the beginning of the GPS week for each satellite. The P code is mixed with the classified W code to get an encrypted Y code. The W code has been empirically found to have a chipping rate of approximately 500 KHz. The Y code is modulated onto the L1 carrier in quadrature with the C/A code and with half the power of the C/A code. The Y code is also modulated onto the L2 carrier signal with half the power of L1 Y code. Both C/A and P codes are unique for each satellite.
GPS receivers are commonly used for a variety of applications involving tracking of the position of various objects. The object to be tracked is coupled to one or more GPS antennae that receive signals from the GPS satellites. Depending upon the level of accuracy and the response time desired by a user, an appropriate method of obtaining position of an object using GPS may be adopted.
Dual frequency receivers that utilize both L1 and L2 frequency signals can determine the position much faster than a single frequency receiver can. A technique that uses both L1 and L2 carrier phase measurements is faster than the one using just L1 carrier phase measurements.
The L1 carrier can be recovered by using any standard correlation technique as the C/A code is known for each of the satellites. The L2 carrier signal is encrypted, thus only military GPS receivers that are aware of the W code can accurately reconstruct the L2 carrier signal. Civilian receivers can also reconstruct the L2 carrier signal using any of the known standard techniques, most of which derive the L2 carrier using the L1 carrier. However, the signal to noise ratio (SNR) of the resulting L2 signal is lower than that of the L2 signals reconstructed using military receivers.
A better way of estimating the phase of the L2 carrier signal is to remove an estimated W code from the L2 signal before phase determination. However, this requires estimation of the unknown W code for the civilian GPS receivers. Various ways of estimating the phase of the L2 carrier signal using the W code have been proposed in the prior art.
U.S. Pat. No. 5,576,715 titled xe2x80x9cMethod and Apparatus for Digital Processing in a Global Positioning System Receiverxe2x80x9d, granted to Leica Inc., Buffalo, N.Y., describes one way of determining the phase of the carrier signal using the W code. The estimated W code is used to track the P code in this patent. The L1 and L2 signals are correlated with locally generated P codes to obtain baseband signals. The baseband signals are separately integrated. The quadrature errors thus produced are integrated over a period of time (approximately the chip period of the W code), which is then used as a control input to adjust the locally generated L2 carrier phase.
Another method of determining the phase of carrier signal is described in U.S. Pat. No. 5,293,170 titled xe2x80x9cGlobal Positioning System Receiver Digital Processing Techniquexe2x80x9d, granted to Ashtech Inc., Sunnyvale, Calif. An estimate of W code obtained from the L1 signal is removed from the L2 signal and the estimate of W code obtained from the L2 signal is removed from the L1 signal. The signal thus obtained allows local oscillators and locally generated estimates of P code to be phase locked with L1 and L2 signals.
Yet another method for determining phase of the carrier signal is described in U.S. Pat. No. 6,125,135 Titled xe2x80x9cSystem And Method For Demodulating Global Positioning System Signalsxe2x80x9d, granted to Navcom Technology, Inc., Redondo Beach, Calif. This patent describes a method of adjusting the locally generated estimate of P code signal in accordance with the estimated W code to obtain better signal strengths. The estimated W code is thereafter multiplied with the quadrature component of the L2signal to obtain an error signal that provides an estimate of the L2 carrier phase.
Another dual frequency GPS receiver is described in U.S. Pat. No 5,736,961 titled xe2x80x9cDual Frequency Global Positioning Systemxe2x80x9d, granted to NovAtel Inc., Calgary, Canada. This patent describes a method of cross correlation in which the L1 signal is correlated using the C/A code phase and the L2 P code phase is determined using the L1 P code phase. A standard feedback loop is used to refine the phase of the L2 carrier signal.
Yet another method as described in U.S. Pat. No. 5,541,606 titled xe2x80x9cW-Code Enhanced Cross Correlation Satellite Positioning System Receiverxe2x80x9d, granted to Trimble Navigation Limited, Sunnyvale, Calif., discloses a W code enhanced cross-correlation technique. Separate estimates of the Y code are generated for the L1 and the L2 signals. The method generates separate W code estimates for both the RF signals by removing the respective P codes from these Y code estimates.
All the abovementioned methods have one or more of the following disadvantages. Some of the methods allow the estimated W code signal to be removed only from the quadrature components of the L1 signal or the L2 signal. All the above methods require generation of a near exact replica of the Y code carrier signal. This requirement results in a need to continuously track the errors and drive them to zero.
Hence, there is a need for a method and a system for estimating the W code that allows its removal from the inphase as well as the quadrature components of the L1 and/or the L2 signals.
An object of the present invention is to provide a W code estimate derived from the L1 signal and the L2 signal. This W code estimate can be removed from the quadrature as well as the inphase components of the L1/L2 signals.
Another object of the present invention is to provide different W code estimates for the L1 signal and the L2 signal. Both the W code estimates are generated using the L1 signal and L2 the signal.
Yet another object of the present invention is to provide an estimate of the W code derived from both the L1 signal and the L2 signal that is uncorrelated with the inphase and the quadrature errors of the L1 and the L2 baseband signals.
Another object of the present invention is to provide the phases and the relative magnitudes of the L1 Y code carrier and the L2 Y code carrier.
To achieve the foregoing objects and in accordance with the purpose of the present invention as broadly described herein, the present invention provides a method and a system for determination of the phase of the L2 Y code carrier. This is achieved by estimating separate W codes for the L1 and L2 signals. The W code estimate for the L1 signal is uncorrelated with the errors in the inphase and quadrature components of the L1 signal. Similarly, the W code estimate for the L2 signal is uncorrelated with the errors in the inphase and quadrature components of the L2 signal.
The RF signals received by the antenna from the GPS satellite are down converted, filtered and sampled to generate first complex signals. The first complex signal comprises two components: an inphase component and a quadrature component. An estimate of P code and an estimate of the carrier are removed from the first complex signals to obtain baseband signals. In the preferred embodiment, the baseband signals thus obtained are accumulated using separate accumulators. The accumulated signals are then weighted and passed through a non-causal FIR LPF. In the preferred embodiment, a one-W-code-bit Integrate and dump (IandD) filter is used. The initial estimates of W code are obtained by subtracting the corresponding weighted signals from the output of the IandD filter. Separate W code estimators act on the initial estimates to generate uncorrelated W code estimates. The L1 W code estimate is uncorrelated with the error in the inphase and the quadrature components of the L1 signal. Similarly, the L2 W code estimate is uncorrelated with the error in the inphase and the quadrature components of the L2 signal. The signals obtained after removing the corresponding estimates of the W code from the baseband signals are then filtered to generate a measure of the magnitude and the phase of the L1 and the L2 signals. In an alternate embodiment, a triangle filter is used instead of an IandD filter. In another alternate embodiment, a single W code estimate is generated for both the L1 and the L2signals.
The present invention provides W code estimates that are uncorrelated with the errors in the inphase and the quadrature components of the L1 signal and the L2 signal. This allows for removal of the W code from the inphase component of both the L1 signal and the L2 signal with zero mean errors. The present invention also provides estimates of relative magnitudes of the L1 signal and the L2 signal, which may be used to estimate the statistics of the phase errors due to noise. The strength of the signals may be maximized later to align the code phase replicas with the incoming signals. The multipath environment may also be studied by looking for alternating periods of constructive/destructive interference. The present invention also obviates the need to generate an exact replica of the Y code carrier before removing it from the L1 and the L2 signals. The present invention also has many implementation advantages because of the simplicity of design.